The present invention relates to a process for preparing 1,2-propanediol, in which a glycerol-containing stream, especially a stream obtained on the industrial scale in the production of biodiesel, is subjected to a low-pressure hydrogenation.
Diminishing mineral oil reserves and rising fuel prices are leading to a growing interest in replacing fuels produced on a mineral oil basis with inexpensive alternatives which do not harm the environment. Processes for producing fuels from biogenic fat- or oil-containing starting mixtures and, for example, used oils and animal fats obtained in restaurants have been known for some time, and predominantly rapeseed oil is currently being used as a starting material in the production of biogenic fuels in central Europe. Biogenic oils and fats themselves are less suitable as motor fuel, since they have to be purified beforehand by usually complicated processes. These include removal of lecithins, carbohydrates and proteins, the removal of so-called oil sludge, and the removal of the free fatty acids present in relatively large amounts, for example, in the rapeseed oil. Vegetable oils thus reprocessed nevertheless deviate from the technical properties of conventional diesel fuels in several aspects. For instance, they generally have a higher density than diesel fuel, the cetane number of rapeseed oil is lower than that of diesel fuel, and the viscosity is several times higher compared to that of diesel fuel. This leads to an unacceptable deterioration in the fuel properties, such as to inhomogeneous running performance of the engine, significantly increased noise emission and, as a result of the higher viscosity, poorer atomization and combustion in the combustion chamber. The use of pure vegetable oils therefore leads to carbonization in conventional engines, associated with elevated particle emission. To solve these problems, it is known that the triglycerides present in the biogenic oil and fat starting mixtures (fatty acid esters of glycerol) can be converted to fatty acid monoalkyl esters, especially methyl or ethyl esters. These esters, which are also known as “biodiesel”, can generally be used in diesel engines without any great modifications and the emission of uncombusted hydrocarbons and soot particles can in many cases even be reduced compared to normal diesel fuel. In the transesterification of the triglycerides for biodiesel production, glycerol is also obtained (≈10%), which should be sent to a utilization both for reasons of economic viability and of sustainability. There is therefore a need for effective and economically viable processes which also enable utilization of the glycerol obtained in the biodiesel production. These processes should more particularly also be suitable for utilizing further glycerol streams available on the industrial scale.
U.S. Pat. No. 2,360,844 describes a process for producing soaps, in which a crude glyceride is transesterified with C1-C4 alkanols and the glycerol released is removed from the monoalkyl esters. The utilization of the glycerol obtained is not described.
U.S. Pat. No. 5,354,878 describes a process for preparing lower alkyl esters of higher fatty acids with a low residual glycerol content by transesterifying fatty acid triglycerides, and the use of these esters as diesel fuel.
DE 102 43 700 A1 describes an ambient pressure process for preparing alkyl esters of higher fatty acids, especially biodiesel, from fatty acid triglyceride starting mixtures comprising free fatty acids with a combination of acidic esterification and basic transesterification. The glycerol obtained in the transesterification is used partly as an azeotroping agent in the esterification of the free fatty acids.
It is known that polyfunctional alcohols can be converted to low-functionality alcohols by catalytic hydrogenation. For instance, DE-C-524 101 describes such a process in which substances such as glycerol are subjected to a gas phase hydrogenation in the presence of a hydrogenation catalyst with hydrogen in a considerable excess. Specifically, for the hydrogenation of glycerol, Cr-activated copper or cobalt catalysts are used.
DE-C-541 362 describes a process for hydrogenating polyoxy compounds, for example glycerol, in the presence of catalysts at elevated temperatures above 150° C. and under elevated pressure. Specifically, the hydrogenation of glycerol with a nickel catalyst at a temperature of from 200 to 240° C. and a hydrogen pressure of 100 atm is described.
R. Connor and H. Adkins describe, in J. Am. Chem. Soc. 54, 1932, p. 4678-4690, the hydrogenolysis of oxygen-containing organic compounds, including that of 98% glycerol, to 1,2-propanediol in the presence of a copper-chromium-barium oxide catalyst.
C. Montassier et al. describe, in Bulletin de la Société Chimique de France 1989, No. 2, p. 148-155, studies of the reaction mechanism of the catalytic hydrogenation of polyols in the presence of various metallic catalysts, for example of glycerol in the presence of Raney copper.
J. Chaminand et al. describe, in Green Chem. 6, 2004, p. 359-361, the hydrogenation of aqueous glycerol solutions at 180° C. and 80 bar of hydrogen pressure in the presence of supported metal catalysts based on Cu, Pd and Rh.
DE 43 02 464 A1 describes a process for preparing 1,2-propanediol by hydrogenating glycerol in the presence of a heterogeneous catalyst at pressures of from 20 to 300 bar, especially at from 100 to 250 bar, and temperatures from 150° C. to 320° C., wherein glycerol is passed over a catalyst bed in vaporous or liquid form. The catalysts mentioned include copper chromite, copper zinc oxide, copper aluminum oxide, and copper silicon dioxide. The use of glycerol-containing streams from biodiesel production and measures for pretreating such streams before their use for hydrogenation are not described in this document.
EP 0 523 015 describes a process for catalytically hydrogenating glycerol to prepare 1,2-propanediol and 1,2-ethanediol in the presence of a Cu/Zn catalyst at a temperature of at least 200° C. In this process, the glycerol is used as an aqueous solution having a glycerol content of from 20 to 60% by weight; the maximum glycerol content in the working examples is 40% by weight.
WO 2005/095536 describes a low-pressure process for converting glycerol to propylene glycol, in which a glycerol-containing stream having a water content of at most 50% by weight is subjected to a catalytic hydrogenation at a temperature in the range from 150 to 250° C. and a pressure in the range from 1 to 25 bar.
M. A. Dasari et al. describe, in Appl. Catalysis A: General 281, 2005, pp. 225-231, a process for low-pressure hydrogenation of glycerol to propylene glycol at a temperature of 200° C. and a hydrogen pressure of 200 psi (13.79 bar) in the presence of a nickel, palladium, platinum, copper or copper chromite catalyst. Different reaction parameters were tested, including the water content of the glycerol used. Although the conversion increased with decreasing water content, the highest selectivity was achieved in this low-pressure process at a water content of 20% by weight.
U.S. Pat. No. 5,616,817 describes a process for preparing 1,2-propanediol by catalytically hydrogenating glycerol at elevated temperature and elevated pressure, in which glycerol having a water content of at most 20% by weight is converted in the presence of a catalyst which comprises from 40% to 70% by weight of cobalt, if appropriate manganese and/or molybdenum and a small content of copper of from 10 to 20% by weight. The temperature is within a range of from about 180 to 270° C. and the pressure within a range of from 100 to 700 bar, preferably from 200 to 325 bar.
The unpublished PCT/EP2007/051983 describes a process for preparing 1,2-propanediol, in which a glycerol-containing stream is subjected to a hydrogenation in the presence of a heterogeneous copper catalyst at a temperature of from 100 to 320° C. and a pressure of from 100 to 325 bar.